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Commit 578e1c4d authored by Junaid Shahid's avatar Junaid Shahid Committed by Paolo Bonzini
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kvm: x86: Avoid taking MMU lock in kvm_mmu_sync_roots if no sync is needed 



kvm_mmu_sync_roots() can locklessly check whether a sync is needed and just
bail out if it isn't.

Signed-off-by: default avatarJunaid Shahid <junaids@google.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 5ce4786f

arch/x86/kvm/mmu.c

+67 −8
Original line number Diff line number Diff line
@@ -2702,6 +2702,45 @@ static bool mmu_need_write_protect(struct kvm_vcpu *vcpu, gfn_t gfn, 
		kvm_unsync_page(vcpu, sp);

	}



	/*

	 * We need to ensure that the marking of unsync pages is visible

	 * before the SPTE is updated to allow writes because

	 * kvm_mmu_sync_roots() checks the unsync flags without holding

	 * the MMU lock and so can race with this. If the SPTE was updated

	 * before the page had been marked as unsync-ed, something like the

	 * following could happen:

	 *

	 * CPU 1                    CPU 2

	 * ---------------------------------------------------------------------

	 * 1.2 Host updates SPTE

	 *     to be writable

	 *                      2.1 Guest writes a GPTE for GVA X.

	 *                          (GPTE being in the guest page table shadowed

	 *                           by the SP from CPU 1.)

	 *                          This reads SPTE during the page table walk.

	 *                          Since SPTE.W is read as 1, there is no

	 *                          fault.

	 *

	 *                      2.2 Guest issues TLB flush.

	 *                          That causes a VM Exit.

	 *

	 *                      2.3 kvm_mmu_sync_pages() reads sp->unsync.

	 *                          Since it is false, so it just returns.

	 *

	 *                      2.4 Guest accesses GVA X.

	 *                          Since the mapping in the SP was not updated,

	 *                          so the old mapping for GVA X incorrectly

	 *                          gets used.

	 * 1.1 Host marks SP

	 *     as unsync

	 *     (sp->unsync = true)

	 *

	 * The write barrier below ensures that 1.1 happens before 1.2 and thus

	 * the situation in 2.4 does not arise. The implicit barrier in 2.2

	 * pairs with this write barrier.

	 */

	smp_wmb();



	return false;

}
@@ -3554,7 +3593,7 @@ static int mmu_alloc_roots(struct kvm_vcpu *vcpu) 
		return mmu_alloc_shadow_roots(vcpu);

}



static void mmu_sync_roots(struct kvm_vcpu *vcpu)

void kvm_mmu_sync_roots(struct kvm_vcpu *vcpu)

{

	int i;

	struct kvm_mmu_page *sp;
@@ -3566,14 +3605,39 @@ static void mmu_sync_roots(struct kvm_vcpu *vcpu) 
		return;



	vcpu_clear_mmio_info(vcpu, MMIO_GVA_ANY);

	kvm_mmu_audit(vcpu, AUDIT_PRE_SYNC);



	if (vcpu->arch.mmu.root_level >= PT64_ROOT_4LEVEL) {

		hpa_t root = vcpu->arch.mmu.root_hpa;



		sp = page_header(root);



		/*

		 * Even if another CPU was marking the SP as unsync-ed

		 * simultaneously, any guest page table changes are not

		 * guaranteed to be visible anyway until this VCPU issues a TLB

		 * flush strictly after those changes are made. We only need to

		 * ensure that the other CPU sets these flags before any actual

		 * changes to the page tables are made. The comments in

		 * mmu_need_write_protect() describe what could go wrong if this

		 * requirement isn't satisfied.

		 */

		if (!smp_load_acquire(&sp->unsync) &&

		    !smp_load_acquire(&sp->unsync_children))

			return;



		spin_lock(&vcpu->kvm->mmu_lock);

		kvm_mmu_audit(vcpu, AUDIT_PRE_SYNC);



		mmu_sync_children(vcpu, sp);



		kvm_mmu_audit(vcpu, AUDIT_POST_SYNC);

		spin_unlock(&vcpu->kvm->mmu_lock);

		return;

	}



	spin_lock(&vcpu->kvm->mmu_lock);

	kvm_mmu_audit(vcpu, AUDIT_PRE_SYNC);



	for (i = 0; i < 4; ++i) {

		hpa_t root = vcpu->arch.mmu.pae_root[i];
@@ -3583,13 +3647,8 @@ static void mmu_sync_roots(struct kvm_vcpu *vcpu) 
			mmu_sync_children(vcpu, sp);

		}

	}

	kvm_mmu_audit(vcpu, AUDIT_POST_SYNC);

}



void kvm_mmu_sync_roots(struct kvm_vcpu *vcpu)

{

	spin_lock(&vcpu->kvm->mmu_lock);

	mmu_sync_roots(vcpu);

	kvm_mmu_audit(vcpu, AUDIT_POST_SYNC);

	spin_unlock(&vcpu->kvm->mmu_lock);

}

EXPORT_SYMBOL_GPL(kvm_mmu_sync_roots);